a. Field of the Invention
The present invention relates to electrical filtering apparatuses used to filter the power input to an electrostatic precipitator.
b. Description of the Related Art
Electrostatic precipitators have taken on considerably greater importance in recent years, particularly in view of the increased emphasis upon maintaining a clean environment. That increased importance includes the need for more effective air pollution control by maintaining clean exhausts from industrial processes that involve either the combustion of fuels or the reaction or transformation of materials in chemical processing operations that result in the generation of particulate matter as a consequence of carrying out the process. The techniques and structural elements incorporated in modern electrostatic precipitators, particularly the electrical control apparatus for controlling the power provided for imparting a charge to the particulate matter to be collected, as well as the power provided to the collection surfaces, have been continually refined to more completely remove undesirable particulate materials from stack gases and also to provide longer useful operating life for the precipitator components. The stack gases in connection with the electrostatic precipitators are often necessary to meet environmental regulations include chemical process and cement plant exhaust gases, fossil fuel electric generating plant exhaust gases, and exhaust gases from steam generation boilers, such as those commonly associated with paper mills for processes such as paper web drying, where scrap "black liquor" from wood processing operations and other fossil fuels are often the fuel sources.
The theory behind the operation of an electrostatic precipitator involves the generation of a strong electrical field through which stack gases pass, so that the particulates carried by the stack gases can be electrically charged. By charging the particles electrically they can be separated from the gas stream and collected, and thereby not enter and pollute the atmosphere. The generation of such electrical fields requires electrical power supplies that can provide a high DC voltage to charge the particulate matter and thereby permit its collection. The existing systems are most often based upon AC corona theory, using a single phase transformer-rectifier (T/R) set to rectify AC power to DC power and provide a high DC potential between a charging electrode and a collection surface, usually a plate, to charge the particles by subjecting the stack gases to the maximum possible current without complete breakdown. That approach is believed to produce the maximum efficiency in effecting removal of such particles.
The emphasis in particulate removal is generally placed on increasing the current flow between a grid and a plate defining the electrostatic precipitator collection surfaces, to a current level that produces a maximum of sparking between the grid and the plate. In fact, some precipitators incorporate a grid structure that contains barbed wire such as DRAGON'S TEETH electrodes or special pointed rods, specifically to enhance such sparking. The sparking inside a precipitator is believed to be necessary as an indicator that the maximum possible current is being drawn, and therefore that the maximum possible ionization of the gases and particles is taking place. In fact, the practice of encouraging sparking is emphasized, even though it is known that sparking produces stresses upon the electrical components of the system, it causes increased precipitator maintenance because of the production of agglomerated particles, sometimes called "ash balls" or "klinkers," and it also causes difficulty in insuring that the rappers, which are devices that vibrate the precipitator plates to remove collected particles, are in fact operative and are removing collected particulate material.
A problem that results from operating an electrostatic precipitator as levels at which sparking occurs is the prevention of damaging arcing. An automatic controller for the input power to the T/R set must sense incipient arcing and immediately reduce the voltage on the precipitator collector plate, because any spark can quickly create an arc between the plate and the electrode, with a resultant high current flow. The high current flow can cause severe damage to the precipitator grid or plate. Additionally, arcing can cause the T/R set to fail, it can cause the controller to fail, or it can open the overcurrent protectors that are provided in the incoming power line. Any of those incidents will cause a section of the precipitator to be temporarily off-line, with the resultant undesirable passing of greater amounts of particulates into the atmosphere until the damage to the precipitator has been repaired. Repair can be a matter of minutes, or it can be weeks if the T/R set or controller has to be replaced.
Heretofore, the prevention of arcing has been attempted by providing complicated sensing and control circuits that add expense to the cost for an electrostatic precipitator. Examples of such circuits are shown in U.S. Pat. Nos. 4,290,003, which issued on Sep. 15, 1981, to Philip M. Lanese; 4,390,831, which issued on Jun. 28, 1983, to William Byrd et al.; 4,587,475, which issued on Oct. 19, 1993, to Gugliemo Liberati. However, the presently available circuits, although effective to some degree, still permit sparking and arcing to occur, thereby requiring more frequent maintenance of the precipitator to repair the damage that is caused by such sparking and arcing. Maintenance involves down time for the precipitator, and usually for the system in which the precipitator is installed, thereby increasing the cost for producing the product of the system in which the precipitator is employed.
In many electrostatic precipitators sulfur trioxide or ammonia, or both, must be injected into the gas stream in order to keep the opacity of the stack gases as low as possible. However, the use of such gases is undesirable because of their caustic nature, that over time causes damage to the precipitator and the stack, again necessitating repair and consequent down time of the process or equipment with which the precipitator is employed.
What is needed, therefore, is an electrical filter apparatus for electrostatic precipitators that provides higher output voltage, reduced voltage ripple, and lower high frequency energy, thereby increasing the ability of precipitators to collect particulate emissions from fossil-fueled boilers and reduce stack opacity.